1. Field of the Invention:
The present invention relates to a process for the production of a material intended for the manufacture of components which are subjected to friction.
2. Discussion of the Background
One skilled in the art is aware that components which are subjected to friction must have a certain number of properties. The main properties are as follows:
a low coefficient of expansion and friction, PA1 adequate resistance to seizure and wear, PA1 good machineability, PA1 a good level of performance with respect to the components against which they rub and which are referred to as "opposing components", in such a way as to limit their wear, and PA1 good mechanical strength when the components involved are engine liners.
Unfortunately, in the area of components subjected to friction, most of the conventional aluminum alloys are at a particular disadvantage due to their relatively high coefficient of expansion, their strong tendency to seizure and wear, and their high coefficient of friction.
It is for that reason that, when there has been a desire to use aluminum for the manufacture of components which are subjected to friction, attention has been directed towards aluminum-silicon alloys. Aluminum-silicon alloys have coefficients of expansion which are lower than that of other alloys which makes them attractive for components which move in relation to each other with close and controlled clearance and where the temperature varies in the course of operation.
Moreover, aluminum-silicon alloys have the advantage that, when they contain silicon in a hypereutectic amount, production by casting gives rise to the formation of hard primary crystals of silicon which are distributed in a softer aluminum matrix. This increases resistance to wear and to seizure. However, when such crystals are formed in a cast alloy mass, they are generally coarse and make it difficult to machine components.
It is for that reason that, in their French patent publication No. 2,343,895, the present inventors recommended producing such alloys from powders produced by atomisation. In that way, by virtue of the high rates of cooling achieved in production, it is possible to form silicon crystals which are smaller than 20 .mu.m in size. When these powders are shaped by extrusion or by sintering, they give rise to the formation of components which can be machined more easily and which also have an improved coefficient of friction.
This procedure also lends itself favorably to incorporating into the alloy lubricating products, such as graphite or tin or hardening agents such as for example silicon carbide. And is also possible to increase the mechanical strength of such alloys by adding other additive elements such as copper, magnesium, etc.
Subsequently, in their French patent publication No. 2,528,910, the inventors proposed novel improvements in the components produced with such alloys, by the powder metallurgy method. They found in fact that such atomised silicon-aluminum alloys contained a large proportion of silicon particles of smaller size than 5 .mu.m. And that the latter, because of their high degree of fineness, were detrimental to good compatibility as between relatively moving components. To solve that problem, the inventors set forth the teaching of using mixtures of powders of aluminum and grains of silicon which were carefully calibrated in a granulometry range of between 20 and 50 .mu.m.
More recently new composite materials, which are still produced from aluminum alloy powders, but which incorporate products referred to as "ceramic" have appeared. The term "ceramic" includes in particular corundum, silicon carbide and zirconia. Such materials are set forth in Japanese patent application No. 59/38350 which claims a sintered aluminum alloy containing 0.5 to 30% of solid lubricant (lead, graphite, molybdenum or tungsten sulphide, copper sulphate and boron nitride); 0.2 to 20% of a hard phase (SiO.sub.2, Al.sub.2 O.sub.3, ZrO.sub.2, and carbides and nitrides of the same elements); 0.2 to 20% of hardening elements (copper, magnesium, silicon, tin and zinc); and 0.2 to 20% of Fe, Ni or Cr to improve resistance to wear.
It is known that the presence of a hard phase makes it possible to improve the resistance of materials which are subjected to friction to wear and seizure, with respect to hypersilicon alloys. But as a correlative aspect these materials give rise to poor machineability and a very high coefficient of friction. This results in the rapid wear of the opposing component. It is for that reason that in the above-noted Japanese patent application such materials were reserved for the manufacture of components such as, e.g., brake shoes and not engine liners, for the manufacture of which they are completely contra-indicated.
There is therefore a strongly felt need for a process for the production of an aluminum alloy-based material which can be used to produce components for friction applications. There is a strongly felt need for such materials which could be easily machined, which would not produce the rapid wear of the opposing component, and which would be resistant to seizure.